The invention relates to a process for the production of pigment/plastic premixes.
As is generally known, titanium dioxide is the most frequently used pigment in the pigmenting of various synthetic products. It can be applied in the forms referred to as rutile or anatase modifications. It can also, however, subsequently be surface treated with SiO.sub.2 or Al.sub.2 O.sub.3. In addition to titanium dioxide, other inorganic, oxidic pigments such as cobalt blue and cobalt violet also find general application.
A condition for the full development of pigment characteristics in a polymer matrix as well as for disturbance-free processing of the pigment polymer compositions, is the complete destruction of the pigment agglomerates and the homogeneous distribution of the pigment particles in the polymer. This is particularly important in the production of thin coatings or foils (generally between 25-50.mu. thickness), since agglomeration of pigment may disadvantageously lead to the formation of holes, and thus to the tearing of the foils. A good dispersion is therefore necessary to avoid this problem.
For the pigmentation of polyolefin compositions therefore, pigments are not applied as such, but rather in form of pigment/plastic premixes, the so-called "master batches", in which the pigment is present in a form already dispersed. The production of such master batches takes place in a special operational process. Typically suited for this purpose are kneading machines, roller mills and extruders.
According to the presently existing state of the art, various methods for the production of master batches have been proposed (for example, U.S. Pat. No. 4,650,747; JP 60-75 832). All methods are characterized in that they require the use of dispersing agents (for example, salts or esters of higher fatty acids, such as stearates) and the treatment of the titanium dioxide surface by means of organic or inorganic agents (such as, for example, alkyl titanate, alkanolamine, alkylpolysiloxane, and zircoaluminate).
The above-stated methods are disadvantageous in that, despite the use of dispersing agents and other auxiliary agents, complete homogeneous distribution of the pigment particles is not achieved.
The treatment of the pigment particles with various auxiliary agents seems, above all, to be inferior. Evidently, the pigment particles are only incompletely encased by the auxiliary agents and, at least in part for this reason, optimal dispersion is not attained. Pigment agglomerates that remain become noticeable in the extrusion coating, for example by a high sieve residue, obstruction of the nozzles, or holes and tears in the resulting film.
Furthermore, the agglomeration of dispersed particles sets a limit to the pigment concentration in the composition. As a result, the desired or necessary concentrations of pigments can frequently not be attained.
One further disadvantage lies in the application of the dispersing agents themselves. These can exude out of a coating and precipitate on the surface of the foil or film. This results in difficulties in the further processing of the polyolefin coatings or polyolefin parts.
It is generally known that a polymerization can take place on the surface of a polymer body or solid substance, resulting in the encasement of the body or substance. See, for example, the encasement of glass beads with polyacrylonitrile (N. W. Johnston et al, Polym. Prep. Amer. Chem. Soc. Div., Polym. Chem., Vol. 17, number 2, 1976, page 491). In this case, the polymerization is initiated by means of radical initiators.
In the attempt to transfer such a process to pigments, such as, for example TiO.sub.2, most particles of the pigment were encased by the polymer. However, the disturbances already mentioned in the extrusion coating caused by high portions of pigment agglomerates appeared to be unchanged. In order to obtain a satisfactory distribution of the pigments, usual quantities of auxiliary agents had to be employed.